1. Field of the Invention
The invention pertains to the generation of symbols in real time on a display screen. The typical application that can be made thereof is the generation of symbols for assistance in air piloting and navigation (e.g., the display of an earth-sky representation with a rotating horizon and an indication of the attitude of the aircraft, the display of a flight plan, a heading dial or card, scrolling altimetrical scales, speed, and various alphanumerical information). These symbols are generated using a symbol generator. The symbol generator is a computer that receives information elements that are needed as a function of the display to be made. For example, information from sensors distributed in the aircraft is used to produce coordinates of successive points of a (colored) trace or plotted line and is displayed on the screen. These coordinates and colors are placed in an image memory. The image memory is read to generate point-by-point information for the control of the display screen so that the desired image appears.
2. Description of the Background
Based on space constraints, there is a trend towards the replacement of cathode-ray tube displays by flat-screen screens, especially liquid-crystal screens. These screens are matrix type screens with corresponding problems. The matrix screens tend to reproduce notched lines and adversely affect the observer's viewing comfort, as the eye is particularly sensitive to sudden transitions in the plotted line from one point to another on the screen.
Accordingly, attempts have been made to smooth plotted lines. This smoothing operation essentially includes plotting relatively thick lines, each line having a width of several rows or of columns around the theoretical line to be plotted, with a distribution of luminance within the line, namely the central part of the line is brighter than the edges of the line. This smoothing operation considerably reduces the unpleasant impression caused by the matrix nature of the screen.
The French patent 87 12039, dated Aug. 28, 1987, and the French patent 90 10587, dated Aug. 23, 1990 define the basic features of this smoothing operation, including replacing each point of the theoretical plotted line by a micro-zone of N.times.M points around the theoretical point and supplying, to the image memory, the luminance values and color values (generally known as the "coefficient of membership in the plotted line") for all the points of a widened plotted line resulting from the succession of the micro-zones along the theoretical single-pixel width plotted line.
FIG. 1a shows a theoretical single-pixel width plotted line including a succession of points Pt1, Pt2, etc. FIG. 1b shows a widened plotted line resulting from the replacement of each point Pt of the single-pixel width plotted line by a micro-zone of 4 points by 4 points, namely a 16-point matrix centered on the point Pt as shown. FIG. 1c shows (1) a micro-zone, now assigned a modulation of luminosity (represented by a variable hatching density) and (2) the smoothed plotted line that results from this modulation of luminosity applied to the micro-zones.
However, this smoothing implies substantial computing capacity: the symbol generator should provide plotted lines computed at a fast rate so that the images are refreshed permanently as a function of the changes undergone by the data elements to be displayed. High-speed computation circuits need to be interposed between the symbol generator and the image memory and for each new point of the plotted line given by the generator, the luminance values (and possibly the color values) of the surrounding points need to be recomputed even if they have already been computed during the plotting of the previous point. These luminance values need to be recomputed when taking into account the values previously stored in the image memory. This results from the overlapping of the micro-zones with respect to one another. There is a correlation to be made between the successive computations performed for one and the same point of the screen. This means that it is necessary to access the image memory with a far greater frequency than would be the case if only one point on the plotted line were displayed. Now, there are physical limits to the possibilities of addressing large-capacity memories (such as an image memory) in read and write mode.
This is why the French patent 90 10587 has proposed a special architecture using as many different memories, addressed in parallel, as there are points (N.times.M) in the micro-zones, in order to make it possible to read and write the totality of a micro-zone in a single step.
This approach may be satisfactory if the size N.times.M of the micro-zone is not too great. However, once this size increases, it becomes impossible to make the smoothing computation circuit in the form of an integrated circuit for it must comprise an extremely large number of inputs/outputs for the addressing of the image memory. It is hardly possible to make 4 point by 4 point micro-zone with a single smoothing integrated circuit for an image with 1024 by 1024 points.
Using micro-zones with larger sizes (e.g., 6.times.6 and even up to 12.times.12) is important since:
(1) it may be useful to represent particularly thick plotted lines for certain symbols without the symbol generator being required to plot several lines in parallel, which would slow down its operation; PA1 (2) certain types of smoothing of plotted lines require a greater micro-zone width than others, as a function of the nature of the contrast desired between the color of the line and the color of the background; and PA1 (3) wider micro-zones enable lines to be plotted with a variable width. It is possible to set the peripheral coefficients of the micro-zone to zero to plot lines of smaller width. This is important because experience shows that colored lines must be plotted wider than white lines. PA1 means for the sending, to the image memory, the coefficients of membership in the plotted line solely for the points of the group G' that do not belong to the group G, as a function of the vector connecting the point X', Y' to the point X, Y, PA1 means for computing the coefficients of membership of the points of the group G based on the coefficients previously computed for the points common to the groups G and G', and PA1 means for the preserving, in a memory of N.times.M cells, with a view to a forthcoming computation, the coefficients of membership computed for the points of the group G. PA1 determining and storing, in a smoothing circuit, a first set of points of a group G' that do not belong to a second set of points of forming group G; PA1 transmitting, to the image memory, first real coefficients computed for the first set of points during the previous cycle and stored in the smoothing circuit; PA1 computing second real coefficients for the second set of points based on instructed-value coefficients and on the first real coefficients computed for the first set of points of the group G' and stored in the smoothing circuit; PA1 storing, in the image memory, for use in a next computation cycle, the second real coefficients computed for all the points of the group G.